Earth’s Interior & Plate Tectonics — Chapter review · Earth & Space Science, Grade 9

Crust, mantle, outer core, inner core.

Starting at the surface and going down: the crust (thin and rocky), then the mantle (a thick layer of hot, slow-flowing rock), then the liquid outer core (iron and nickel), then the solid inner core (iron and nickel) at the very center.

The crust and the uppermost part of the mantle.

The lithosphere is defined by physical property (rigid and brittle), not composition, and it includes the crust plus the very top of the mantle, all riding together as one rigid shell broken into tectonic plates.

3,420 km combined; about 50% of Earth's radius.

Add the two thicknesses: 2,200 + 1,220 = 3,420 km. Divide by Earth's radius: 3,420 ÷ 6,371 ≈ 0.537, or about 54%, which rounds to the nearest 10% as about 50% — roughly half of Earth's radius.

The immense pressure at Earth's center keeps the iron-nickel atoms packed too tightly to flow, even at extreme temperature.

Whether iron stays solid or turns liquid depends on both temperature and pressure, not temperature alone. The inner core is indeed hotter than the outer core, but the pressure from the entire weight of the planet pressing down on it is so enormous that it forces the iron and nickel atoms to stay locked in a solid structure, unable to flow the way the less-pressurized outer core can.

They move apart; example: the Mid-Atlantic Ridge (or the East African Rift).

Divergent boundaries pull plates apart, letting magma rise up to form new crust. The Mid-Atlantic Ridge, where the ocean floor is spreading and slowly widening the Atlantic Ocean, is a classic example.

A volcanic island arc, such as Japan.

When two oceanic plates converge, the denser one subducts beneath the other, melts, and feeds a curving chain of volcanoes that rises up out of the ocean as a volcanic island arc — Japan is a well-known example.

Neither continental plate is dense enough to subduct, so the crust crumples and folds upward into mountains instead of melting into magma.

Subduction (and the volcanoes it feeds) only happens when one plate is dense enough to sink beneath the other. Continental crust is too light on both sides, so instead of sinking, the colliding edges buckle and fold upward, building mountain ranges like the Himalayas with little to no volcanic activity.

About 1,000 times more ground-shaking.

Each whole number on the magnitude scale represents about 10× more shaking. From magnitude 4 to 7 is three steps, so 10 × 10 × 10 = 1,000 times more shaking.

The station with the 6-minute gap is farther from the epicenter.

P-waves always travel faster than S-waves, so the gap between their arrival times grows the farther the waves travel. A bigger gap (6 minutes) means the waves traveled a longer distance, so that station is farther from the epicenter than the one with only a 1-minute gap.

A hot spot — a stationary plume of hot mantle rock melts through the crust while the plate slowly slides over it, leaving a trail of progressively older volcanoes.

This pattern is the signature of a hot spot: a plume of unusually hot rock rises from deep in the mantle and stays in roughly the same place while the tectonic plate above it slowly slides across. Each time the plate shifts, the hot spot burns through a new spot in the crust, building a fresh volcano, while the older volcanoes are carried away from the hot spot and gradually go extinct — creating a chain that gets older with distance from the currently active volcano, just like the Hawaiian Island chain.

It suggests those continents were once joined together, since Glossopteris could not have spread its seeds across a vast ocean.

Glossopteris was a land plant with seeds too heavy to be carried across entire oceans by wind or water. Finding its fossils on continents that are now far apart only makes sense if those continents were once connected, allowing the plant to spread across a single connected landmass before the continents drifted apart.

Wegener had no mechanism for how continents could move; seafloor spreading and paleomagnetic stripes discovered in the 1960s provided that mechanism.

Wegener could show strong evidence that continents had once been joined, but he could not explain a physical process powerful enough to push solid continents through solid ocean floor, so most scientists dismissed the idea. That changed in the 1960s, when scientists discovered seafloor spreading at mid-ocean ridges, confirmed by symmetric stripes of paleomagnetism recorded in the ocean floor rock — finally giving continental drift, as part of plate tectonics, a real mechanism.

a liquid outer core

S-waves cannot cross liquid, so their absence marks a liquid layer.

The crust is about half a percent of Earth's radius, so at true scale it would be a line barely thicker than the ink, and the mantle would dominate the picture. Textbook diagrams exaggerate the crust so its features can be labelled, which is useful but gives a misleading impression of how thin it actually is.

Response criteria

• State a scientifically accurate conclusion.
• Show the data, mechanism, or calculation that supports it.
• Include units, conditions, or limits where relevant.

Lithosphere is a mechanical layer including crust and rigid uppermost mantle, so it overlaps compositional categories. Most mantle is solid and can deform slowly under sustained stress; convection does not establish liquid state.

Lithosphere is a mechanical layer including crust and rigid uppermost mantle, so it overlaps compositional categories. Most mantle is solid and can deform slowly under sustained stress; convection does not establish liquid state.

Response criteria

• Use the stated measurements or conditions, with correct units.
• Connect the result to the physical mechanism or evidence relationship.
• Identify the assumption, model limit or further evidence requested.

a subduction zone

The trench marks where the descending plate bends downward.

Both result from the interaction of plates, and those interactions happen at plate edges rather than across plate interiors. Mapping earthquake and volcano locations therefore traces the boundaries themselves, which is one of the strongest lines of evidence that the surface is divided into moving plates.

Response criteria

• State a scientifically accurate conclusion.
• Show the data, mechanism, or calculation that supports it.
• Include units, conditions, or limits where relevant.

The gap closes at 6−2 = 4 cm/year. Relative convergence is established, but plate types, trench geometry, earthquake depths and geological evidence are needed to distinguish subduction from other convergent settings.

The gap closes at 6−2 = 4 cm/year. Relative convergence is established, but plate types, trench geometry, earthquake depths and geological evidence are needed to distinguish subduction from other convergent settings.

Response criteria

• Use the stated measurements or conditions, with correct units.
• Connect the result to the physical mechanism or evidence relationship.
• Identify the assumption, model limit or further evidence requested.

100

Each unit is a tenfold amplitude increase, so two units is 10 × 10 = 100 times.

P-waves travel faster than the slower, more damaging S-waves and surface waves, so detecting the P-wave arrival near the source gives a short interval — seconds to tens of seconds — before the strong shaking reaches a more distant place. That interval is enough to stop trains and shut valves, though not to evacuate, and it shrinks to nothing close to the epicentre.

Response criteria

• State a scientifically accurate conclusion.
• Show the data, mechanism, or calculation that supports it.
• Include units, conditions, or limits where relevant.

The S–P gap is 36 s. Distance inference needs the appropriate travel-time model; direction requires other stations. Damage also depends on local intensity, depth, ground and structures, so one gap or surface location is insufficient.

The S–P gap is 36 s. Distance inference needs the appropriate travel-time model; direction requires other stations. Damage also depends on local intensity, depth, ground and structures, so one gap or surface location is insufficient.

Response criteria

• Use the stated measurements or conditions, with correct units.
• Connect the result to the physical mechanism or evidence relationship.
• Identify the assumption, model limit or further evidence requested.

Matching fossils and rock sequences across oceans, symmetric magnetic stripes with ages increasing away from mid-ocean ridges, and present-day plate velocities measured by satellite positioning. Each could be questioned alone — fossils by land bridges, for instance — but they come from different fields with different weaknesses, and a single alternative would have to explain all three at once, which is what makes the conclusion secure.

Response criteria

• State a scientifically accurate conclusion.
• Show the data, mechanism, or calculation that supports it.
• Include units, conditions, or limits where relevant.

20 m

2 × 1,000 = 2,000 cm = 20 m.

80/4 = 20 km per million years = 2 cm/year on one side; full symmetric separation is 4 cm/year. Check opposite-side age patterns and magnetic records; variable rates or asymmetric movement can challenge the assumption.

80/4 = 20 km per million years = 2 cm/year on one side; full symmetric separation is 4 cm/year. Check opposite-side age patterns and magnetic records; variable rates or asymmetric movement can challenge the assumption.

Response criteria

• Use the stated measurements or conditions, with correct units.
• Connect the result to the physical mechanism or evidence relationship.
• Identify the assumption, model limit or further evidence requested.

A location estimate with uncertainty; travel-time models and measurements are not exact. Additional stations may narrow the region.

The distance defines an approximate set of possible source locations around the station, not a unique direction. The retrofit changes vulnerability, while local physical hazard is not necessarily changed. Additional stations and verified building-performance evidence are needed.

The distance defines an approximate set of possible source locations around the station, not a unique direction. The retrofit changes vulnerability, while local physical hazard is not necessarily changed. Additional stations and verified building-performance evidence are needed.

Response criteria

• Use the stated measurements or conditions, with correct units.
• Connect the result to the physical mechanism or evidence relationship.
• Identify the assumption, model limit or further evidence requested.